Palladium(II) Complexes of Aminopolycarboxylic Ligands in Aqueous Solution

Published: October 04, 2011

r 2011 American Chemical Society 4759 dx.doi.org/10.1021/je200759g | J. Chem. Eng. Data 2011, 56, 4759–4771

ARTICLE

pubs.acs.org/jced

Palladium(II) Complexes of Aminopolycarboxylic Ligands in AqueousSolutionConcetta De Stefano,† Antonio Gianguzza,*,‡ Alberto Pettignano,‡ and Silvio Sammartano†

†Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universit�a di Messina, V. le Ferdinando Stagno D’Alcontres31, I-98166, Messina (Vill. S. Agata), Italy‡Dipartimento di Chimica “Stanislao Cannizzaro”, Universit�a di Palermo, Viale delle Scienze, I-90128, Palermo, Italy

bS Supporting Information

ABSTRACT:The binding capacity of five aminopolycarboxylic ligands (APCs) [nitrilotriacetate (NTA), ethylenediamine-N,N,N0,N0-tetraacetate (EDTA), (S,S)-ethylenediamine-N,N0-disuccinic acid (S,S-EDDS), diethylenetriamine-N,N,N0,N00,N00-pentaacetate(DTPA), and triethylenetetraamine-N,N,N0,N00,N000,N000-hexaacetate (TTHA)] toward the palladium(II) ion was studied bypotentiometric titrations (ISE-H+ electrode) in NaNO3 and in NaClO4/NaI (at different molar ratios) solutions and byspectrophotometric titrations (only in NaClO4), at I = 0.1 mol 3 kg

�1 and at T = 298.15 K. The high stability of Pd2+-complexonesspecies inhibits the formation of sparingly soluble hydroxo species until pH 11. In the pH range investigated and at the Pd2+/APCratios used in the experiments (1/1�1/4), all of the complexones form with palladium(II) only protonated or unprotonatedmononuclear species and the monohydroxo species Pd(APC)(OH). As consequence of the ligand structure and of the number ofamino and carboxylic groups involved in the coordination, the stability of palladium(II)�APC species follows the trend: TTHA >DTPA > EDTA > S,S-EDDS > NTA; for example, the log KPdL is 37.00, 36.31, 23.60, 23.07, and 17.82 for L = TTHA, DTPA,EDTA, S,S-EDDS, and NTA, respectively. A critical evaluation of literature evidenced the big discrepancies in stability data and alsoin the speciation models proposed by different authors. A comparison of the sequestering capacity of complexones towardpalladium(II) and other divalent metal ions was done by using stability data from this work and literature ones.

1. INTRODUCTION

The more and more increasing use of catalytic converters toreduce the dangerous NOx and CO emission in the atmosphereby vehicle traffic produces a corresponding increase of palladiumin the environment,1 this metal, together with platinum and rho-dium (the so-called platinum group elements, PGE), being themain component of the catalytic converters in the automobiles.Among the PGE, elemental palladium seems to be the mosthazardous one because it can be easily and quickly oxidized topalladium(II) when put in contact with soil.2,3 The presence ofpalladium oxidized forms is of great concern since an increase ofthe mobility and the bioavailability of this element in the environ-ment and an easier uptake by plants, animals, and humans canoccur. On the other hand, the environmental presence of palla-dium as palladium(II) ion could be an advantage in setting-upsoil and water remediation processes if appropriate chelatingagents for this metal ion removal are found.

Since more than 50 years, the most used chelating agentsare the (poly)amino (poly)carboxylic ligands (APCs, also called“complexones”).4 APCs, which include at least 20 differentmolecules, are employed in several application fields, for exam-ple: (i) in agriculture to modulate the metal absorption by rootsand to removemetals from contaminated sites,5,6 (ii) inmedicinefor chelation therapy7 and radio diagnostic analysis,8 (iii) in watertreatment to remove undesirable and toxic metal ions,9 and (iv)in all of the other industrial processes (photography, cosmetics,pulp and paper, detergents, food, metal plating, etc.) where metalions must be absent (or present at negligible concentration) in

the final products.10 The most used complexones are nitrilotria-cetate (NTA), ethylenediamine-N,N,N0,N0-tetraacetate (EDTA),and diethylenetriamine-N,N,N0,N00,N00-pentaacetate (DTPA) asdocumented by their utilization in Europe of about 200 000 tons inthe year 2000.4

The great success of complexones as metal chelating agents isdue to the very high stability of the complex species formed insolution which allows the considerable reduction of the activity offree metal ions in solution. Almost all metal ions form with APCscomplex species with different stability, as documented by severalarticles and compilations where critically evaluated values ofstability constants are also reported.11�13 Unfortunately, fewdata are reported in the literature on the stability of the speciesformed by interaction of complexones with platinum groupelements (PGE, platinum, palladium, rhodium), and no criticalvalue is reported for the relative stability constants. The lack ofstability data for complex species formed in these systems is incontrast with the increase of the worldwide demand of PGEs.

With the aim to evaluate the possibility of using complexonesas chelating agents for the palladium(II) ion in aqueous solution,we report here stability data for the interaction of Pd2+ with the fol-lowing APC ligands: NTA, (S,S)-ethylenediamine-N,N0-disuccinic

Special Issue: Kenneth N. Marsh Festschrift

Received: July 19, 2011Accepted: September 20, 2011

4760 dx.doi.org/10.1021/je200759g |J. Chem. Eng. Data 2011, 56, 4759–4771

Journal of Chemical & Engineering Data ARTICLE

acid (S,S-EDDS), EDTA, DTPA, and triethylenetetraamine-N,N,N0,N00,N000,N000-hexaacetate (TTHA) (structures are shown inFigure 1).

The choice of these complexones depends on the followingfactors: (i) NTA, EDTA, DTPA, and TTHA were chosen toevaluate the influence of the increasing number of amino andcarboxylic donor groups on the stability of complex species;(ii) the S,S-EDDS was chosen because it has been shown to be themost biodegradable among the complexones. This is of particularrelevance from the environmental point of view. In fact, most ofcomplexones have low biodegradability and maintain their struc-ture and their reactivity unaltered after they have been largelyused in metal removal processes. As a consequence some undesi-rable effects can be produced, such as (i) remobilization of metalions from sediments into the natural waters (lakes, rivers, sea-waters, etc.); (ii) changing of uptake of essential and toxic metalions by plants and other living organisms.4 Results obtained bybio and/or photo degradation tests, expressed as readily biode-gradability indices (RBI), show that EDTA is the less biodegrad-able APC, while the highest biodegradability is shown by S,S-EDDS.14�17 The S,S-EDDS is one of the three stereoisomers ofEDDS (S,S-EDDS, S,R/R,S-EDDS, and R,R-EDDS), which, ascan be seen from Figure 1, has two chiral carbons in themolecule.S,S-EDDS is produced by microorganisms, and for the firsttime it has been isolated as a product of the metabolism of theAmycolatopsis orientalis actinomycete in soils.17�19 Among thethree stereoisomers only S,S-EDDS is almost completely bio-degradable as confirmed by several tests used to evaluate the

permanence time of polymeric materials in the environment.20,21

It is selectively produced by the reaction between 1,2 dibromo-methane and L-aspartic acid.15,22,23 Since it shows high biode-gradability and no toxicity, S,S-EDDS can be considered anenvironmentally friendly chelating agent that could substitute theother APCs in all of the application fields.

For the above reasons S,S-EDDS was included among theAPCs to be considered as chelating agents for the palladium(II)ion. Investigations were performed to assess the complex speciesformed in each Pd2+�APC system and to determine the relativestability constants with the aim to quantitatively define thesequestering capacity of each APC ligand toward the palladiumion in aqueous solution.

From the analysis of the literature stability constant values forthe NTA, (S,S)-EDDS, EDTA, DTPA, and TTHA�Pd2+ com-plex species formation24�29 (see Table 1), a discrepancy emergesprobably due to the use of different techniques in determiningthe value of the stability constant of Pd(APC) species which is toohigh to be evaluated only by the classical potentiometric (protondisplacement) technique.

To overcome the difficulty in the calculation of ML speciesstability constants, the results obtained from two series of ISE-H+

potentiometric titrations (in NaNO3 and in mixed NaClO4/NaIionic medium) were combined with those obtained from ISE-H+

potentiometric/spectrophotometric titrations (in NaClO4). Mea-surements were carried out at T = 298.15 K and at ionic strengthI = 0.1 mol 3 kg

�1 in the different (single and mixed) ionic media(details on the experimental conditions used are reported in the

Figure 1. Structure of aminopolycarboxylic ligands.



experimental section). The stability constants determined here arediscussed and compared with those reported in the literature. Thestability data obtained for the species formed in the differentPd2+�APC systems allowed us to assess the different sequesteringcapacity of the five complexones used toward the palladium(II)ion in the pH range investigated. To this end the pL50 parameter,already tested successfully for other metal�ligand systems,30�33

was used. This parameter gives quantitative information on thesequestration capability of a ligand toward metal ions at fixedexperimental conditions of pH and ionic strength. Details on themeaning and calculation of pL50 are reported in the sequesteringcapacity section. In our opinion the pL50 parameter can be used asa valid alternative to the speciation efficiency indices (SEI) alreadyproposed for industrial purposes.10,34

2. MATERIALS AND METHODS

2.1. Chemicals. Palladium(II) nitrate dihydrate (by Sigma-Aldrich, purum) stock solutions were prepared by dissolving theweighed salt in 0.1 mol 3 kg

�1 nitric acid. The palladium concen-tration was checked by the inductively coupled plasma atomicemission spectroscopy (ICP-AES) technique against Pd(NO3)2standard solutions (Fluka)35 or gravimetrically using the di-methylglyoxime method.36 NTA, S,S-EDDS, DTPA, and TTHAwere used in their acidic forms; EDTA was used as disodium salt.All ligands were supplied by Fluka with analytical grade purity,and their purity was checked potentiometrically by alkalimetrictitrations. NaNO3, NaClO4, and NaI solutions were prepared byweighing pure salts (Fluka) after drying in an oven at 383.15 Kfor 2 h. Nitric acid, perchloric acid, and sodium hydroxide solu-tions were prepared by diluting concentrated ampules (Riedelde Ha€en) and were standardized against sodium carbonate and

potassium hydrogen phthalate, respectively. NaOH solutionswere preserved from atmospheric CO2 by means of soda limetraps. All solutions were prepared with analytical grade water(R = 18 MΩ) using grade A glassware.2.2. Potentiometric Apparatus and Procedure. Potentio-

metric titrations were carried out at (298.15 ( 0.1 K) using anapparatus consisting of a model 713 Metrohm potentiometer,equipped with a combined glass electrode (Ross type 8102, fromOrion) and a model 765 Metrohm motorized buret. The esti-mated accuracy was( 0.2 mV and( 0.003 mL for electromotiveforce (emf) and titrant volume readings, respectively. The appa-ratus was connected to a PC, and automatic titrations wereperformed using a suitable computer program to control titrantdelivery and data acquisition and to check for emf stability. Alltitrations were carried out under magnetic stirring, and presatu-ratedN2 was bubbled through the purified solution to excludeO2

and CO2 inside. Five series of potentiometric titrations wereperformed to study the formation of the complex species ofpalladium(II) with the complexones investigated (NTA, S,S-EDDS, EDTA, DTPA, and TTHA) in NaNO3 and in mixedNaClO4/NaI ionic medium, at I = 0.1 mol 3 kg

�1. In the case ofmeasurements in mixed ionic medium, several NaClO4/NaIratios were used. In all titrations, the titrand solutions containknown amounts of palladium(II) nitrate, aminopolycarboxylicligand, supporting electrolyte, and the necessary amount of HNO3

or HClO4 to acidify the solution at pH ≈ 1. In each series oftitrations, a volume of 25 mL of the titrand solution was titratedwith standard NaOH. Details of the experimental measurementsare reported in Table 2. About 80 to 100 points were collectedfor each titration, and the equilibrium state during titrations waschecked by monitoring the time necessary to reach equilibrium

Table 1. Literature log β Values a of Pd2+�L Species (L = NTA, S,S-EDDS, EDTA, DTPA or TTHA) in Different Ionic Media,Ionic Strengths, and Temperatures

medium Ib Tc

log

β110

log

β111

log

β112

log

β113

log

β114 ref

Pd2+�TTHA

NaClO4 0.5 25 18.73 25.65 28.55 31.05 33.50 25

(6.92) (2.90) (2.50) (2.45)

Pd2+�DTPA

NaBr/NaClO4 0.2/0.8 20 29.7d 33.2 36.26 38.99 41.29 29e

(3.50) (3.06) (2.73) (2.3)

Pd2+�EDTA

NaClO4 1 20 24.5d 27.51 29.82 30.72 29f

(3.01) (2.31) (0.9) (<0)

ClO4� 0.2 25 18.5 26

not reported 0.1 25 26.4 28

NaClO4 0.2 21 25.6 27

Pd2+�S,S-EDDS

KNO3 0.1 30 13.6 24

Pd2+�NTA

NaClO4 1 20 17.1 19.58 23.7 g 29

(2.48) (6.6) g

a log βpqr refers to eq 1; log Kpqr reported in parentheses refers to eq 2b Inmol 3L

�1. c In �C. d logβ110 value calculated inNaBraq at I= 1mol 3L�1. eOther log

Kplr values at different NaBr/NaClO4 ratio are reported by authors.f log β110 = 25.5 in KBr, at I = 1 mol 3 L

�1 and T = 293.15 K was reported by authors.g log β120 and log K120.



(several titrations were carried out with different equilibrationperiods ranging between (10 and 60) s per data point) and byperforming back-titrations. For each experiment, independenttitrations of strong acidic solution with standard base were car-ried out under the same temperature, medium, and ionic strengthconditions as in the systems to be investigated, with the aim ofdetermining the electrode potential (E0) and acidic junctionpotential (Ej = ja [H

+]).2.3. Spectrophotometric Apparatus and Procedure. The

UV�vis spectra were recorded in the wavelength range (250 to400) nm, using a spectrophotometer (Beckman, DU 640B)equipped with a peristaltic pump (Velp Scientifica, type SP 311)and a flow cell (Hellma, 176.000-QS) with a 1 cm optical pathlength. The spectrophotometer was connected to a PC for the

acquisition of the spectra. The combined potentiometric andspectrophotometric titrations were carried out by using the samecell and potentiometric apparatus described in the previous para-graph. A volume of 40 mL of the solutions containing the com-plexone, palladium(II) ion, perchloric acid, and sodium perchlo-rate at I = 0.1 mol 3 kg

�1 was titrated with a standard NaOHsolution to obtain the maximum absorbance of each predomi-nant species, according to the preliminarly determined speciationdiagrams of the systems. Details of experimental measurementsare reported in Table 2.2.4. Calculations. The nonlinear least-squares computer

program ESAB2M37 was used for the refinement of all para-meters of the acid�base titration (E0,Kw, liquid junction potentialcoefficient, ja, analytical concentration of reagents). The BSTACand STACO38 and HYPERQUAD 200639 computer programswere used in the calculation of complex formation constants frompotentiometric titrations. UV�vis spectra were analyzed by theHYPSPEC40 program, which allows us to calculate stability con-stants and the molar absorbance of each absorbing species, usingas input the experimental absorbances, analytical concentrationsof reagents, and the chemical model proposed. The ES4ECI41

program was used to draw speciation diagrams and to calculatespecies formation percentages. The LIANA42 program was usedto fit different linear and nonlinear functions.Overall and stepwise protonation constants of complexones,

hydrolysis constants of palladium(II) ion and formation con-stants of Pd2+�APC complex species are given according to theequilibria 1 and 2 (p = 0 for protonation; q = 0 and r negative forhydrolysis):

pPd2þ þ qLz� þ rHþ ¼ PdpLqHrðqz � 2p � rÞ� βpqr

ð1Þ

PdpLqHr�1ðz � r þ 1Þ� þ Hþ ¼ PdpLqHr

ðqz � 2p � rÞ� Kpqr

ð2Þwhere Lz� = NTA3�, S,S-EDDS4�, EDTA4�, DTPA5�, orTTHA6� and negative r values stand for OH�. Formationconstants, concentrations, and ionic strengths are expressed inthe molal (mol 3 kg

�1) concentration scale [formation constantsin molar (mol 3 L

�1) concentration scale are reported as supple-mentary data; in many cases the difference between the concen-trations in the two scales (molar and molal) is not significant andwithin the experimental errors].

Table 2. Experimental Conditions for Potentiometrica andSpectrophotometric Measurements at I = 0.1 mol 3 kg

�1 and T= 298.15 K

technique ionic medium CPdb CL

b pH range

NTA

potentiometry (ISE-H+) NaNO3 0.8�1.0 1.0�2.2 2.05�11.10

spectrophotometry NaClO4 0.4 1.6�2.5 1.40�11.02

S,S-EDDS


potentiometry (ISE-H+) NaClO4/NaIc 0.4�0.5 1.9�2.5 3.79�10.59


EDTA




DTPA




TTHA



spectrophotometry NaClO4 0.4 1.3�1.6 1.60�11.07a 80 to 100 points for each measurement. bAnalytical concentration inmmol 3 kg

�1; cAt different ratios.

Table 3. ProtonationConstantsa ofNTA,b S,S-EDDS, EDTA,cDTPA,b andTTHAb inNaNO3, at I = 0.1mol 3 kg�1 andT = 298.15K

ligand log β011 log β012 log β013 log β014 log β015 log β016 log β017

TTHA 10.492 19.983 26.082 30.019 32.650 34.822 36.371

(9.491)d (6.099) (3.937) (2.631) (2.172) (1.549)

DTPA 10.126 18.636 22.83 25.536 27.666 28.776

(8.510) (4.19) (2.706) (2.130) (1.110)

EDTA 9.36 15.451 18.163 20.306

(6.091) (2.712) (2.143)

S,S-EDDS 9.89 ( 0.02 16.87 ( 0.02 20.82 ( 0.02 23.87 ( 0.02 26.08 ( 0.04

(6.98) (3.95) (3.05) (2.21)

NTA 9.327 11.868 13.573 14.885

(2.541) (1.705) (1.312)a log β0qr refers to eq 1 and are expressed in molal (mol 3 kg

�1) concentration scale; b From ref 45. c From ref 44. d log K0qr in parentheses refers to eq 2.



3. RESULTS AND DISCUSSION

In building the complexation models for all investigatedpalladium(II)�APC systems, the protonation of APC ligands(NTA, S,S-EDDS, EDTA, DTPA, and TTHA) and the hydro-lysis of palladium(II) ion were always taken into account.3.1. Protonation of APC Ligands.The protonation constants

of APCs are in general strongly dependent on the ionic mediumas largely discussed in a previous article.43 The protonationconstants of NTA, EDTA, DTPA, and TTHA used here (seeTable 3) are literature data;44,45 the protonation constants of S,S-EDDS were determined in this work. All of the protonationconstants used in this work are reported at I = 0.1mol 3 kg

�1 (Na+

ionic medium) and at T = 298.15 K.3.2. Hydrolysis of Palladium(II) and Interaction with the

Anions of the Supporting Electrolyte. The high stability ofPd2+�APC complex species, in the pH range investigated,strongly inhibits the hydrolysis of palladium(II) ion; nevertheless,for accuracy, the hydrolysis constants of Pd2+ ion were consideredin the speciation models of all of the investigated systems.The values of the conditional hydrolysis constant of the

Pd(OH)+ species, the most important hydroxo species of palla-dium(II) ion, log KPd(OH) = �2.08 in NaClO4 (considered bythe majority of authors as a noninteracting medium) and logKPd(OH) =�2.22 in NaNO3 ionic media (I = 0.1 mol 3 kg

�1, T =298.15 K) were taken from literature (ref 32 and referencestherein).The interaction of the palladium(II) ion with the anions of the

supporting electrolytes was also taken into account. The forma-tion constant values of the PdIn species reported by Elding andOlsson46 at I = 1 mol 3 L

�1 (see Table 4) were used in thecalculations to obtain formation constants at I = 0.1 mol 3 kg

�1 byusing the extended Debye�H€uckel type eq 3:

log K ¼ log TK � z 3 0:51I1=2ð1 þ 1:5I1=2Þ�1 þ CI þ DI3=2

ð3Þ

where C and D are empirical parameters and z* = ∑-(charges)reactants

2 � ∑(charges)products2 . K is the formation con-

stant and TK the formation constant at infinite dilution. In Na+

media, according to previous results (unpublished results fromthese laboratories), the values C = 0.067z* and D = �0.04z*,can be used. In the mixed perchlorate�iodide medium thelog KPd(OH) obtained in NaClO4, at the corresponding ionicstrength, was considered.3.3. Complex Species Formation in the Pd2+�APC Sys-

tems.The complex species formation refers to equilibria 1 and 2.In the calculations, the formation of simple, protonated, andmixed hydroxo species has to be considered, under the experi-mental concentrations used (0.5 e CPd/mmol 3 kg

�1 e 1 and1 e CAPC/mmol 3 kg

�1 e 5) and in the pH range (2 to 11.5)investigated.

As pointed out before, the stability constant of the PdL species(L = APC) is too high to be determined by classical potentio-metric ISE-H+ measurements carried out in noninteracting ionicmedia, such as NaNO3 or NaClO4. To overcome this difficulty,we followed a different approach based on potentiometric mea-surements in mixed interacting and noninteracting ionic mediaand on spectrophotometric measurements. First of all, an accu-rate check of literature data was made selecting, when possible,the most reliable values of the logKPd(APC) for each system inves-tigated.These values were used, in the first approximation, to perform

calculations on the experimental potentiometric data obtainedfrom titrations of Pd2+�APC solutions (APCs =NTA, EDTA, S,S-EDDS, DTPA, and TTHA) in NaNO3 noninteracting ionicmedium. In particular the following formation constant valuesfrom the literature data were used: log KPdNTA = 17.1, logKPdEDTA = 26.4, and log KPdDTPA = 29.7.28,29 In the case of S,S-EDDS and TTHA ligands, the log KPd(APC) values of 13.6 and18.73 (the only data present in literature) reported by Sunar et al.and by Napoli, respectively,24,25 were considered too low and notconsistent with those of the other Pd2+�complexone systems.For this reason we decided to use for S,S-EDDS the same for-mation constant as for EDTA and for TTHA, the logKPd(TTHA) =37.2. The log KPd(TTHA) value was obtained by using theempirical eq 4 for the dependence of Pd2+�APC formationconstants (APC = NTA, EDTA, DTPA, IDA)29 on the numberof amino (nN) and carboxylic (nCOOH) groups and on theirdistance (expressed as the number of �CH� between thesegroups, d) in the APC ligand molecule:

log KPdðAPCÞð ( 0:6Þ ¼ p1 þ p2nN2 þ p3nCOO þ p4d

ð4ÞwhereKPd(APC) is the stability constant of the Pd

2+�APC speciesand p1, p2, p3, and p4 are empirical parameters. From the stabilitydata used for the Pd(APC) species in the different systemsconsidered we found: p1 = 12.8, p2 = 0.8, p3 = 1.7, and p4 =�0.7.By using these values for the Pd�TTHA system (nN = 4, nCOOH =6, and d = 1), the value of log KPdTTHA = 37.2 was calculated.These stability data for all of the Pd(APC) species in the

different Pd2+�APC systems were considered as a baseline for

Table 4. Iodo Complexes Species of the Palladium(II) Ion inNaClO4/NaI Mixed Ionic Medium, at T = 298.15 K

I

ionic medium mol 3 kg�1 log KPdI log βPdI1 log βPdI3 log βPdI4

NaClO4/NaI 1 6.06a 22a 25.7a 28.2a

NaClO4/NaI 0.1 6.37b 22.43b 26.23b 28.58b

aReference 46. bThis work (calculated).

Figure 2. Experimental UV spectra of Pd2+�NTA complex speciesrecorded in aqueous solutions containing: CPd = 4.5 3 10

�4 mol 3 kg�1,

CNTA = 1.6 3 10�3 mol 3 kg

�1, I = 0.1 mol 3 kg�1 in NaClO4, at T =

298.15 K.



the next two steps of our investigation in which ISE-H+ poten-tiometric titrations in NaClO4/NaI mixed ionic medium andspectrophotometric/ISE-H+ potentiometric titrations in NaClO4

ionic medium were carried out at I = 0.1 mol 3 kg�1. By both

potentiometric and spectrophotometric/potentiometric measure-ments, it was possible to calculate the log KPd(APC) values and tovalidate the accuracy of the log β values of the Pd2+�APCdifferently protonated or hydroxo species for all of the complex-ones considered. In fact, as results from potentiometric measure-ments carried out in NaClO4/NaImixed ionicmedium, the iodideion forms a very stable complex species with the palladium(II) ion(log KPdI = 6.08, log βPdI2 = 22, log βPdI3 = 25.8, log βPdI4 = 28.3 inNaClO4/NaI, at I = 1 mol 3L

�1);46 the formation of the Pd2+�Ispecies leads to a reliable lowering of the formation percentage ofPd2+�APC species and allows the calculation of the formationconstants of ML species in all of the Pd2+�APC systems. Thereliability of complexationmodels of Pd2+�APC systems obtainedby ISE-H+ potentiometry was confirmed by spectrophotometrictitrations. In fact, all of the species, including the protonated andthe hydroxo ones, formed in the Pd2+�APC systems show intensebands in the wavelength range (250 to 400) nm. As an example,theUV spectra of the Pd2+�NTAcomplex species in the pH range1.4 to 11.64 and the molar extinction coefficients, εmax, in thewavelength range (250 to 500) nm are reported in Figures 2 and 3.Spectrophotometric investigations have been performed con-

sidering the following points: (i) the complexones (differentlyprotonated) do not absorb in the spectral range investigated,(ii) the Pd2+ ion and its hydrolytic species normally absorb atthese wavelengths (for Pd(H2O)4

2+ λmax = 379 nm, εmax =78 L 3mol

�13 cm

�1),47 (iii) in the pH range considered only theabsorption of Pd2+�APC species results, while the absorption ofboth the Pd2+ aquo ion and the palladium hydrolytic species iscompletely suppressed; (iv) as a consequence, in the data pro-cessing by Hypspec computer program,40 the aquo ion andhydrolysis species of palladium(II) were not included in thelist of absorbing species identified by factor analysis. Table 5collects the maximum values of molar extinction coefficients,εmax (L 3mol

�13 cm

�1) of the Pd2+�APC species and the corres-ponding wavelengths, λmax (nm), obtained by the deconvolutionof spectra determined in solutions containing different Pd2+�APC concentration ratios, as reported in Table 2.Finally, the potentiometric titration data in NaNO3 ionic

medium were reanalyzed by keeping constant the values of logK(ML) in the complexation models for all of the Pd2+�APC

systems. The results obtained by following this approach arereported in Table 6 together with the formation constants of allof the species obtained by potentiometry and spectrophotometryin the different ionic media.As can be seen, simple PdL species, mononuclear protonated

species, PdLqHr(2�zq+r) (the r value depends on the APC con-

sidered), andmixed hydroxo species, PdL(OH), are formed in allthe systems investigated. Moreover, in the Pd2+�NTA systemthe Pd(NTA)2 species is formed too.In general, APCs form very stable species with Pd2+ ion in the

entire pH range (2 to 11.5) investigated, including the acidic pHrange. For this reason, in spite of the high stability of hydrolyticspecies of palladium(II), which begin to be formed at low pHvalues (∼2), the formation of simple hydrolytic species of themetal ion in the presence of APC ligands is suppressed in thewhole pH range (2 to 11.5) investigated, as shown in the distri-bution diagrams reported in Figure 4 (CPd = 1mmol 3 kg

�1,CAPC =2 mmol 3 kg

�1, in Na+ medium, at I = 0.1 mol 3 kg�1 and at T =

298.15 K).The stability of the Pd(APC) species follows the trend: TTHA>

DTPA > EDTA > S,S-EDDS > NTA. As expected, the stabilityis a function of the number of binding groups (�NHand�COOH)present in the complexone molecule. In the case of S,S-EDDSand EDTA, which are structural isomers having the same number

Figure 3. Molar extinction coefficients, ε (L 3mol�13 cm

�1), of Pd2+�NTA complex species at T = 298.15 K.

Table 5. Spectroscopic Data for Pd2+�APCComplex Species

λmax εmax

species nm L 3mol�13 cm

�1

TTHA

Pd(TTHA)H3 325 976 ( 5

Pd(TTHA)H2 325 1210 ( 6

Pd(TTHA)H 325 1330 ( 4

Pd(TTHA) 330 1080 ( 5

Pd(TTHA)(OH) 330 1070 ( 4

DTPA

Pd(DTPA)H3 325 1010 ( 35

Pd(DTPA)H2 325 1130 ( 29

Pd(DTPA)H 325 1390 ( 26

Pd(DTPA) 325 1410 ( 12

Pd(DTPA)(OH) 325 1330 ( 30

EDTA

Pd(EDTA)H2 345 642 ( 6

Pd(EDTA)H 335 1040 ( 6

Pd(EDTA) 335 1270 ( 4

Pd(EDTA)(OH) 360 640 ( 13

S,S-EDDS

Pd(S,S-EDDS)H2 325 1070 ( 12

Pd(S,S-EDDS)H 330 1200 ( 10

Pd(S,S-EDDS) 330 1310 ( 10

Pd(S,S-EDDS)(OH) 330 1350 ( 26

NTA

Pd(NTA)H 345 323 ( 5

Pd(NTA) 370 564 ( 4

Pd(NTA)(OH) 355 361 ( 8

Pd(NTA)2 330 433 ( 7



of amino and carboxylic binding groups, the following other factorsmust be considered in evaluating the relative stability of complexspecies: (i) the nature of amino groups, which are tertiary in theEDTA and secondary in S,S-EDDS, and (ii) the distance betweenbinding groups in the molecule.3.4. Sequestering Ability of APCs toward the Palladium(II)

Ion and Other Divalent Metal Ions. The stability data obtainedfor all of the species in the different Pd2+�APC systems wereused to evaluate the sequestering capacity of the complexonesunder investigation toward the palladium(II) ion. Moreover,since the natural waters and wastewaters are multicomponentsolutions where different metal ions can be present simulta-neously, the sequestering capacity of the same ligands was alsoevaluated, for comparison, toward other metal ions (Cd2+, Cu2+,Hg2+, Pb2+, Ca2+, Mg2+) of environmental and biological interest.This investigation is useful to assess the sequestration capabilityof the complexones (i) in multicomponent solutions, such asnatural waters and wastewaters, where different metal ions can be

present simultaneously and can compete in metal complexformation with the ligands also present, (ii) in some industrialapplications where the choice of the best sequestering agent toremove undesirable metal ions is often important, and (iii) inagricultural practices where complexones can be used as seques-tering agents in the chelate-assisted metal phytoextraction.18,48,49

For the above reasons, it can be very useful to have a parameterable to give immediate information on the sequestering ability ofa ligand (in our case APCs) toward metal ions in the conditions(ionic medium composition, ionic strength, pH, metal ionconcentration, etc.) of the medium where the metal sequestra-tion occurs (e.g., contaminated natural waters, wastewaters, orwaters used in a particular industrial process). The sequesteringability of a generic ligand is, of course, strictly related to thestability of the complex species formed with the metal ion, butalso to all of the possible competing reactions involving both theligand and the metal ion, that is, on their speciation. Therefore,the knowledge of the stability of the metal�ligand complex

Table 6. Formation Constants of Pd2+�APC (NTA, S,S-EDDS, EDTA, DTPA, and TTHA) Complex Species Obtained withDifferent Techniques, at I = 0.1 mol 3 kg

�1 and at T = 298.15 K

log βpqra,b

species potentiometry (ISE-H+) spectroscopy

ionic medium NaNO3 NaClO4/NaI NaClO4

L = NTA

PdLH 20.25 ( 0.01c (2.44) 20.00 ( 0.01c (2.19)

PdL 17.82 17.82 ( 0.01

PdL2 24.50 (6.69) d 24.50 ( 0.01 (6.69) d

PdLOH 9.99 ( 0.02 (�7.83) 10.41 ( 0.01 (�7.41)

L = EDTA

PdLH2 28.3 ( 0.1 (1.57) 28.55 (2.01) 28.55 ( 0.01 (2.01)

PdLH 26.72 ( 0.01 (3.15) 26.54 (2.94) 26.54 ( 0.01 (2.97)

PdL 23.58 23.61 ( 0.09 23.58 ( 0.01

PdLOH 13.33 ( 0.01 (�10.25) 13.67 ( 0.07 (�9.94) 13.67 (�9.91)

L = S,S-EDDS

PdLH2 28.62 ( 0.01 (1.89) 28.62 (1.89) 28.62 (1.89)

PdLH 26.73 ( 0.01 (3.64) 26.73 (3.64) 26.73 (3.7)

PdL 23.1 23.1 ( 0.1 23.04 ( 0.01

PdLOH 12.00 ( 0.01 (�11.1) 12.00 (�11.1) 12.00 (�11.04)

L = DTPA

PdLH3 45.42 ( 0.01 (2.69) 45.42 (2.69) 45.42 (1.74)

PdLH2 42.73 ( 0.02 (2.7) 42.73 (2.7) 43.68 ( 0.02 (2.9)

PdLH 40.03 ( 0.01 (3.66) 40.03 (3.79) 40.78 ( 0.03 (4.41)

PdL 36.38 36.25 ( 0.02 36.38 ( 0.04

PdLOH 24.95 ( 0.02 (�11.43) 24.87 ( 0.07 (�11.38) 24.3 ( 0.1 (�12.08)

L = TTHA

PdLH3 50.0 ( 0.1 (2.35) 50.0 (2.35) 50.0 (2.35)

PdLH2 47.64 ( 0.03 (3.66) 47.64 (3.72) 47.64 (3.66)

PdLH 43.98 ( 0.01 (6.87) 43.92 ( 0.03 (7.05) 43.98 (6.87)

PdL 37.12 36.88 ( 0.05 37.12 ( 0.01

PdLOH 27.34 ( 0.02 (�9.78) 26.59 ( 0.02 (�10.29) 27.34 (�9.74)a log βpqr refers to eq 1 and are expressed in molal (mol 3 kg

�1) concentration scale; Lz� = NTA3�, S,S-EDDS4�, EDTA4�, DTPA5�, or TTHA6�;negative r values stand for OH�. b Stepwise formation constants logKpqr in parentheses refer to eq 2 or in the case of the PdL(OH) species to PdL

(z�2)�

+ OH� = PdL(OH)(z�1)�. c( Standard deviation. d Stepwise formation constants in parentheses refer to the reaction: Pd(NTA)� + NTA3� =Pd(NTA)2

4�.



species only is not sufficient to assess the efficiency of a seques-tration process, but the speciation picture of both the metal andthe ligand must be also known. An objective representation andquantification of the sequestering ability of a ligand toward a metalion canbe givenby the semiempirical parameter pL50 that representsthe total ligand concentration necessary to bind (sequestrate) the50% of trace metal ion present into the solution. This parameter,already successfully tested for various systems (see, e.g., refs 30�33),is obtained by the following Boltzmann-type equation:

∑% ¼ A1 � A2

1 þ eðpL � pL50Þ=Sþ A2 ð5Þ

where ∑% represents the total percentage of metal complexspecies (Pd2+ or the other divalent metal ions, in our case)

formed with the ligand considered, A1 = 100 (∑% for pL f 0)and A2 = 0 (∑% for pL f �), and S is the curve slope at 50 %complexation, which is always equal to 0.434. In the light of theseconsiderations, eq 5 can be written as:

∑% ¼ 100

1 þ 10ðpL � pL50Þð5aÞ

More details about the pL50 calculation and the method used todraw the sequestration diagrams can be found in ref 33, where acomparison between pL50 and similar parameters is also given.Briefly, the sequestration diagrams are drawn by plotting in they-axis the sum of all of the metal(II) complex species with theligand (APC, in our case) calculated considering different ligandconcentrations andCM2+ = 10�14 mol 3 kg

�1. In this way, the pL50

Figure 4. Distribution diagrams of PdpLHr(z�2p�r)� species vs pH, at I = 0.1mol 3 kg

�1 in NaNO3, atT = 298.15 K; Lz� =NTA3� (a), S,S-EDDS4� (b),EDTA4� (c), DTPA5� (d), and TTHA6� (e). Experimental conditions: CPd = 1 mmol 3 kg

�1, CL = 2 mmol 3 kg�1. p1r indexes in figures refer to

PdpLHr(z�2p�r)� species, and negative r values stand for OH�.



varies with the experimental conditions (pH, ionic strength,supporting electrolyte, temperature, etc.), but it is independentof the analytical concentration of the metal ion. Moreover, sinceall of the side interactions occurring in the system (metal hydro-lysis, ligand protonation, interactions with other components)are taken into account in the speciation model, they are notconsidered in the calculation of pL50 since they do not contributeto the pL50 parameter as previously defined. On the basis of theseconsiderations it can be concluded that the stronger the bindingability of the ligand, the higher the pL50 is. Table 7 shows the pL50values calculated for the sequestration of Pd2+ and the selectedother divalent metal ions by the APCs at pH = 6, at I =0.1 mol 3 kg

�1 and at T = 298.15 K. With the exception of thepL50 values of Pd

2+�APC systems, the sequestration parametersof the complexones toward the otherM2+ ions were calculated byusing hydrolysis and complex formation constants taken fromthe literature.12,50

Figure 5 shows, as an example, the sequestration curves ofEDTA toward all divalent metal ions considered, at pH = 6.0.At this pH the sequestration capability of EDTA toward Pd2+ ionis higher than the one toward the other divalent metal ions.

The same behavior is shown by all APCs considered here, as canbe seen in Figure 6where the pL50 values (seeTable 7) ofNTA, S,S-EDDS, EDTA, DTPA, and TTHA are plotted for all divalent metalions considered at the same pH andmetal concentration conditions.As pointed out before, the sequestering capacity of a ligand

toward ametal ion is strictly dependent on the pH of the mediumwhere the metal sequestration occurs. Figure 7a�e shows thedependence on pH (in the range 2 to 10) of pL50 for each APCligand used toward the metal ions considered.By using the pL50 data, empirical relationships can be found for

the dependence on pH. In particular, the dependence on pH ofpL50 of APC ligands toward Pd2+ ion can be expressed by thefollowing equation:

pL50 ¼ p1 þ p2ðpH� p3Þ2 ð6Þ

where p1, p2, and p3 are empirical parameters whose values arereported in Table 8.3.5. Literature Comparison. Among synthetic chelating

agents the APCs are the most used and, as a consequence, themost studied in terms of their sequestration capability towardmetal ions.12 Analogously, palladium(II), as the other PGE, is theobject of the research of many scientists that deal with speciationstudy in natural systems. Despite the great environmental impor-tance of both APC ligands and palladium(II) ion, in literaturethere are relatively few and dated papers24�29 dealing with thedetermination of complexation ability of the former toward thelatter. The lack of thermodynamic data on palladium(II)�APCcomplexes is probably due to (i) the difficulty in the calcula-tion of hydrolysis constants of palladium(II) ion, that in NaNO3

ionic medium undergoes hydrolysis at pH < 2 and (ii) the highstability constant values of the APCs complex species.All of the literature data on Pd2+�complexones interactions

are reported in Table 1. From their critical analysis emerges thatsome of these data (e.g., stability constants of Pd2+�TTHA com-plex species reported by Napoli,25 the log KPd(S,S‑EDDS) reportedby Sunar et al.24 and the logKPd(EDTA) reported byMacNevin andKriege26) do not meet criteria for their selection by the most impor-tant database for stability constants such as, for example, the NISTdatabase.Moreover,most of the other literature stability constants ofTable 1 are classified by NIST as “tentative” or “provisional” data,12

with the exception of log KPd(DTPA) = 29.7, log KPd(DTPA)H = 3.50,

Table 7. Values of pL50 for the Sequestration of Pd2+ andOther Divalent Metal Ionsa by NTA, S,S-EDDS, EDTA,DTPA, and TTHA, at pH = 6, in NaNO3 Ionic Medium, at I =0.1 mol 3 kg

�1 and T = 298.15 K

pL50

metal ion NTA S,S-EDDS EDTA DTPA TTHA

Pd2+ 10.75 13.90 16.01 25.87 25.89

Cu2+ 9.36 13.30 14.96 14.71 15.80

Pb2+ 8.14 7.77 13.99 12.15 12.26

Zn2+ 7.32 8.66 12.49 11.87 12.17

Cd2+ 6.43 5.17 12.50 12.46 13.52

Hg2+ 5.47 7.11 12.00 14.35 14.93

Ca2+ 2.97 0.70 6.83 4.41 5.00

Mg2+ 2.16 1.30 4.98 3.53 4.32aThe stability constants of hydrolytic species are from ref 50; complexformation constants of M2+�APC species are from ref 12.

Figure 5. Total percentage of M2+ ions complexed (∑%) by EDTAvs �log CEDTA and at T = 298.15 K. Experimental conditions: CM2+ =10�14 mol 3 kg

�1 (trace), pH = 6, I = 0.1 mol 3 kg�1 in NaNO3aq.

Figure 6. pL50 values for divalent metal ions sequestration by APCs(NTA, S,S-EDDS, EDTA, DTPA, and TTHA). Experimental condi-tions: CM2+ = 10�14 mol 3 kg

�1 (trace), pH = 6, in Na+ ionic medium, atI = 0.1 mol 3 kg

�1 and atT = 298.15 K. Symbols:9, EDTA;b, DTPA;2,TTHA; 1, NTA; �, S,S-EDDS.



and logKPd(EDTA) = 24.5 reported byAnderegg andMalik29 that arelabeled as “recommended” stability constants.12

The different experimental conditions (ionic medium, ionicstrength, temperature) together with the low reliability of a con-sistent part of literature stability constants make the comparisonwith our thermodynamic data very difficult.However, by taking into account the different experimental

conditions used by the different authors, the following considera-tions can be made.(i) There is a good agreement between our and literature

values of log K11r. For example, our log K111 values for thePd2+�TTHA, Pd2+�DTPA, Pd2+�EDTA, and Pd2+�NTA systems are: 6.96, 3.96, 3.06, and 2.31, respectively[I = 0.1 mol 3 kg

�1 (Na+ medium), T = 298.15 K]. Theanalogous log K111 of literature are: 6.92 [I = 0.5 mol 3 L

�1

Figure 7. Dependence on pH of pL50 of NTA (a) S,S-EDDS (b), EDTA (c), DTPA (d), and TTHA (e) toward the divalent metal ions. Experimentalconditions: Na+ ionic medium, I = 0.1 mol 3 kg

�1 and T = 298.15 K. Symbols: 0, Ca2+; O, Mg2+; 4, Cu2+; 3, Cd2+; v, Pb2+; w, Zn2+; f, Pd2+.

Table 8. Parameters of Equation 6 for the Dependence on pHof pL50 of APCs towards the Pd

2+ Ion

APC p1 p2 p3 σa

NTA 10.14 0.02 0 0.22EDTA 16.40 �0.1 8.23 0.15S,S-EDDS 14.5 �0.17 7.9 0.50DTPA 28.4 �0.07 11.8 0.33

TTHA 29.1 �0.07 12.7 0.24a Standard deviation on the fit.



(NaClO4), T = 298.15 K],25 3.50 [I = 1 (0.2 + 0.8)mol 3 L

�1 (NaBr/NaClO4 mixed medium), T = 293.15K],29 3.01 [I = 1 mol 3 L

�1 (NaClO4), T = 293.15 K],29

and 2.48 [I = 1 mol 3 L�1 (NaClO4), T = 293.15 K).29

(ii) A rough comparison can bemade between the value of logβPdEDTA = 23.60 [I = 0.1 mol 3 kg

�1 (Na+ medium), T =298.15 K] reported here and the values for the same spe-cies reported by Anderegg et al.29 [log βPdEDTA = 24.5 atI = 1 mol 3 L

�1 (NaClO4), T = 293.15 K) and by Kragtenet al. [log βPdEDTA = 26.4 at I = 0.1 mol 3 L

�1 (mediumnot reported), T = 293.15 K,28 and log βPdEDTA = 25.6 atI = 0.2 mol 3 L

�1 (NaClO4), T = 294.15 K27].(iii) Also the log βPdNTA = 17.82 [I = 0.1 mol 3 kg

�1 (Na+

medium), T = 298.15 K] obtained here is comparablewith the stability constant reported by Anderegg et al.(log βPdNTA = 17.1 at I = 0.1 mol 3 L

�1 (NaClO4), T =293.15 K].29

4. CONCLUSIONS

A systematic study on the binding capacity of five complex-ones toward Pd2+ ion in Na+ ionic media was made by potentio-metric and spectroscopic techniques. Hydrolysis constants ofmetal ions, protonation constants of APC, and, in the case of mea-surements in NaClO4 /NaI ionic medium, formation constants ofPdIn species were all considered in the speciation model ofPd2+�APC systems. The stability of the Pd(APC) species fol-lows the trend: TTHA > DTPA > EDTA > S,S-EDDS > NTA,and as expected, the stability is a function of the number of bind-ing groups (�NH and�COOH) present in the APC molecule,but this trend is not linear. If we consider the homogeneousseries, NTA, EDTA,DTPA, andTTHA, with the general formulaNn(CH2)3n (COOH)n+2 the following sigmoidal equation canbe written for log βPdL as a function of n:

log β ¼ a1� b

1 þ 10ðn � bÞ þ b

� �ð7Þ

with a = 16.73 and b = 2.28. The parameter a refers to thetheoretical minimum log β value (for nf 0) and b is the n valuecorresponding to the maximum slope for the function log βversus n. A similar behavior can be found for log βCuL[I = 0.1 (K+) mol 3 L

�1],12 with a = 12.19 and b = 1.79.The suggested formation constants of Pd2+�APC complex

species at I= 0.1mol 3 kg�1, inNa+ ionicmedium, atT = 298.15 K

are reported in Table 9.The stability data calculated for the Pd2+�APC complex

species were used to quantitatively define, in terms of pL50, the

sequestration capability of APCs toward the palladium(II) ion.Some considerations can be done on the basis of pL50 values:(i) all of the APCs considered strongly sequestrate Pd2+ in the pHrange investigated; (ii) a comparison of pL50 of the complexonesfor different divalent metal ions (Ca2+, Mg2+, Cu2+, Cd2+, Pb2+,Zn2+) evidenced that in the pH range typical of natural fluids (6 to 8)all of the APCs preferentially sequestrate the palladium(II) ion;(iii) although among the APCs considered the S,S-EDDS is not thebest sequestering agent for Pd2+ ion, its high biodegradability and asufficient high pL50 in the pH range investigated make this APC thebest choice when it is necessary to conjugate an environmentalfriendly and a good sequestration treatment.

’ASSOCIATED CONTENT

bS Supporting Information. Protonation constants for theseries (Table 1S), iodo complex species in mixed medium (Table2S), formation constants of the complex series (Table 3S), andsuggested log βpqr values (Table 4S). This material is availablefree of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +39 091489409. E-mail address: [email protected].

’ACKNOWLEDGMENT

We are pleased to contribute to this special issue to celebrateDr. Kenneth N. Marsh for his excellent work in investigating thethermochemical and thermophysical properties of organic com-pounds and for his dedicated engagement as Editor-in-Chief ofthe Journal of Chemical & Engineering Data for many years.

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Palladium(II) Complexes of Aminopolycarboxylic Ligands in Aqueous Solution

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